Hot rolled steel, stainless steel and other metals are currently descaled by a process called pickling. Pickling involves advancing the steel through long acid baths that remove the oxide layers that form scale. Most carbon steel strip is pickled in hydrochloric acid tanks at strip speeds of about 400 to 1000 feet per minute. It is more difficult to remove scale from stainless steel and the descaling process requires stronger acids such as, hydrofluoric, sulfuric, or nitric acid. Pickling of stainless steel also requires longer times in the acid tank which reduces the line speeds for stainless steel strip down to about 100 to 400 feet per minute. The disposal of byproducts resulting from the pickling process is hazardous, as well as costly, as the byproducts are considered to be toxic pollutants.
Conducting the pickling process can also be problematic. Line stops, where the metal strip is stopped in the acid for an extended period of time, often result in overpickling. Overpickling may damage the surface of the metal strip. Different types of metals require varying acid mixtures for optimum pickling. If the same line is being used for multiple types of metal, line stops and changeover time are incurred when the acid mixture is changed. Pickled metal is left with a low pH (less than 7) causing the metal to reoxidize unless protected from oxygen by a layer of oil. The oil is expensive to apply and must be removed for certain downstream processing steps such as painting or coating.
Descaling of metal surfaces can also be performed using two common blasting techniques. A first blasting technique uses relatively large particle shot at low velocities to assist in acid descaling. A second technique descales with a jet of sharp edged abrasive media such as sand, silicon carbide, aluminum oxide, or steel grit. Abrasive jet descaling is somewhat inefficient for two reasons. Continuous descaling of metals, particularly carbon and stainless steel, with abrasive media having sharp edges causes the media to embed itself into the steel surface. Therefore, heavy coverage with the abrasive media is needed to completely clean off the oxide layers. In addition, the embedded sharp edged media must be removed in what is typically a costly and difficult abrading step.
U.S. Pat. No. 6,088,895 to Nelson et al. discloses a method for descaling hot rolled strip that uses shot blasting in conjunction with tension leveling, brush cleaning and brush polishing. In particular, this patent discloses stretching the strip in tension to at least 1% elongation to level the strip and induce cracking in the scale covering the strip, shot blasting using metallic particles propelled from a blasting wheel to ablatively remove a portion of the scale, mechanically removing additional scale to using two pairs of counter-rotating wire brushes until the metal sheet reaches a surface roughness of 3.6 micron Ra, and polishing the strip with another pair of brushes to reduce the roughness to within a range of about 2.0 micron Ra.
It would be advantageous to have a high-speed, rugged mechanical method and apparatus for cleaning scale from metal surfaces that avoids the problems of acid pickling. More particularly, it would be advantageous the have a descaling method and apparatus that requires minimal steps to produce a relatively smooth surface that has been thoroughly cleaned of scale. It would be further advantageous if the surface was resistant to formation of scale after descaling.